Resistors are used in a variety of commercial applications where it is necessary to convert excessive amounts of electrical energy to thermal energy. The thermal energy can be dissipated through the use of a forced cooling mechanism. Subway cars, trains and other large transportation vessels are among the many applications for resistors as described herein.
More particularly, resistors can be used to dissipate electrical energy generated by electrical motors which drive the wheels of large cargo trucks, thus, retarding the speed of the wheels. Conventional trucks are typically propelled by an engine which directly drives the wheels, and pad or disk brakes are used to stop or slow the vehicle. However, it is often necessary, for example, in mining operations, for a truck carrying large loads, i.e., ore, to repetitively carry those loads down steep grades. The disk or pad brakes as used on a conventional truck are quickly worn under such conditions.
Thus, mining trucks are typically outfitted with a diesel engine which drives individual electrical motors to propel individual wheels. As the truck goes down the side of a large hill or mountain the electrical motors retard the movement of each wheel thereby generating large amounts of current. The current is transmitted through electrical conductors to a bank of resistors where the current is transformed to thermal energy. Thermal energy is dissipated through forced cooling of the resistor elements.
Such resistors are often subjected to extremely harsh conditions and must be relatively substantial in construction and design to withstand the harsh operating conditions. Resistors often operate at temperatures of about 500.degree. C. and are frequently used on trucks which traverse unimproved roads, for example, in a mountain mine or logging area. Thus, resistors must be capable of withstanding severe conditions.
Other design considerations must also be taken into account when designing a resistor. For example, it is desirable to maximize the amount of surface area of the resistor elements within each resistor. For a given material of a given thickness, the amount of electrical energy which can be transformed to thermal energy and dissipated increases as the surface area of the resistor element increases. However, it is often necessary to force cool the resistor elements to continuously remove the thermal energy from the surface of the resistor elements. Forced cooling generally requires a gap or spacing between individual panels of each resistor element. Thus, the resistor element within the resistor volume must be carefully designed to maximize surface area of the resistor element while at the same time providing sufficient air flow through the resistor elements for cooling purposes. Additionally, the resistor elements must be properly supported to maintain structural integrity under harsh working conditions.
In the past, individual resistor plates have been used wherein the ends of the resistor plates were connected in series to form a stack of spaced resistor elements. This conventional configuration has at least two major flaws, both occurring at the connection between individual panels. The panels were typically spot welded. This created localized areas of overheating because the connection between the panels was not uniform. Additionally, the current traveling down one element had to make a 180.degree. turn to travel up the second element. This transition occurred in a very short and abrupt manner at the connection of the two panels.
To alleviate the problems associated with individual element panels, some resistor manufacturers have used thin resistor elements bent into a fan fold continuous resistor configuration. The fan fold resistor element eliminated the need for welding or otherwise connecting individual element panels and allowed for a smooth transition from one panel to another. The transition between panels generally occurred over a smooth substantially "U" shaped bend at the end of each element panel. However, the fan fold resistor element configuration proved to be physically unstable. As the length of each individual element panel, i.e., the distance between the "U" shaped bends increased, so did the failure rate of the resistors due to the instability of the individual element panels. The instability was generally caused by movement of the element panels as the element panels reached operating temperatures and became soft and/or pliable. This problem was dealt with in the past by making the length of the individual panel members shorter and supporting the ends of the resistor element bends.
U.S. Pat. No. 4,100,526 which issued to Kirilloff et al., is typical of grid resistors of the past which utilized fan fold resistor elements. As can be seen, the Kirilloff et al. resistor utilizes eight individual fan folded resistor elements. Approximately every other "U" shaped bend between resistor element panels is supported by a bar running between the individual resistor elements. The support bars running between resistor elements are nonconductive and do not provide any resistance, i.e., they are not part of the element and serve only as support. The eight individual resistor elements are connected in series at their ends. Thus, a considerable amount of the resistor space is consumed by support bars which are not resistive, take up considerable space, add weight and are relatively intricate in design and manufacture. Additionally, because the individual resistor elements are connected in series at their ends, there are 16 connections providing multiple points for failure. Also the individual connections are heavier and more substantial than the thin resistor element, thus, the connections also add considerable weight to the overall resistor product.
There has been a continuing need for resistor elements which are light weight, easy and relatively inexpensive to manufacture and utilize elongated element panels to eliminate the need for multiple elements. Additionally, there is a need for the elimination of the supports for the individual bends of the resistor element due to the complexity, and added weight and cost of the bend supports.